João M. Gil

Virtual MIMO schemes for downlink space-frequency coding OFDM systems

In this paper we investigate the performance of single-relay cooperative scheme, where the source is equipped with a 2-antenna array and both the relay and destination with a single antenna. We consider Amplify-and-Forward, Decode-and-Forward and Selective Decode-and-Forward relaying protocols and a space-frequency block coding designed for DL OFDM based systems is employed. Signals expressions are derived for each relay protocol under various scenarios, considering power constraint imposed on cooperating nodes. The aim of this work is to evaluate this Virtual MIMO scheme in realistic scenarios (e.g. correlated antennas, precise channel models, Doppler effects) for implementation purposes in a system with LTE based parameters and considering channel turbo coding. The coded performance of the proposed Virtual MIMO scheme is evaluated, and compared against non-cooperative OFDM based systems, for typical pedestrian scenarios based on LTE specifications. The proposed cooperative scheme outperforms the co-located MISO and MIMO for some scenarios.

Combining directional channel modelling with beamforming adaptive antennas for UMTS

The conjugate gradient algorithm is applied to antenna beamforming at the base-station, in the uplink, for UTRA-TDD. Specific microcell, street type propagation scenarios are used, applying a wideband directional channel model, based on the elliptical single bounce model. The scenarios are also characterised by the placement of a variable number of mobile interfering terminals, at different positions and groupings. Simulation average results mainly cover final beamforming processing gain and signal-to-noise ratio, focusing on their relation to propagation conditions. Algorithm convergence evolution and complexity are summarily addressed. The high dependency of the algorithms performance on the propagation channel and on the number and positioning of the several mobile terminals has been confirmed, establishing such relationships. The average gain values present a minimum value of 7.3 dB, for all scenarios. It has also been concluded that a power control procedure highly improves the beamforming gain, in scenarios where the near-far effect is critical, rising up to 6.9 dB.

Adaptive beamforming dependencies on wideband and directional propagation characteristics in micro- and macro-cell UMTS scenarios

It is important to study the fundamental wideband and directional factors in order to establish how the channel properties affect adaptive beamforming, regarding intra-cell interference reduction. With such aim, a comparative set of micro- and macro-cell propagation scenarios is generated, allowing for the systematic analysis of the involved factors. A non-blind adaptive beamforming algorithm is applied, the conjugate gradient at the base station in the up-link makes use of the mobile terminal identifying the UMTS codes. Performance is measured through the gain in SINR relative to that obtained with a single omnidirectional antenna. The scenarios involve the variation of the grouping and angular displacement of mobile terminals, their distances to the base station, their number, and the number of array elements. It is found that the angular and temporal densities of incoming signals are the fundamental roots behind the differences in performances between macro- and micro-cells. For macro-cells, involving much larger angular densities, the effect of having common angles-of-arrival among links is much more critical than for micro-cells; in the case that terminals are grouped, beamforming gains are close to 0 dB. In such case, for micro-cells, values range from 0.9 to 9.6 dB, for 16 users, with a monotonous dependency on distance and on the number of array elements. Micro-cell environments not only allow for the application of simpler beamforming adaptive implementations, but also render larger performance increases by improving such implementations.

Distributed Space-Time Code Using Precoding for Cellular Systems

We propose a data precoded relay-assisted scheme, for a system cooperating with 2 relays. We assume that all the terminals are equipped with a single antenna. The use of relays is of significant interest to allow radio access in situations where a direct path is not available. However because of the half duplex constraint at the relays, transmission of a data rate equivalent to that of a modulation technique with m bits per symbol in the case a continuous link would be available from the BS to the UT requires the use of a constellation with 2m bits per symbol. This implies a penalty in power efficiency. The proposed simple precoding scheme exploits the relation between QPSK and 16-QAM, by alternately transmitting through the 2 relays, keeping the diversity of order 2. Numerical results show significant performance improvements relatively to the case of a relay-based distributed Alamouti employing 16-QAM, and a performance very close to the non-cooperative system employing 2x1 QPSK Alamouti coding with a continuous link available, when all the links have the same SNR.

Distributed space-frequency block coding for a 2-antenna relay in downlink OFDM systems

In this paper we evaluate the coded and uncoded performance of a 2-antenna terminal relaying schem, for downlink OFDM based systems, using efficient space-frequency block coding protocols. We consider that the relay and the base station are equipped with 2 antennas and the user terminal has a single antenna. Two types of relay protocols are accounted for: Equalize-and-Forward and Decode-and-Forward. Signals expressions are derived and the bit error rates obtained for a pratical and preeminent scenario, with LTE parameters and using convolutional channel coding. The performance of the proposed relay-assited scheme is evaluated under realistic scenarios, considering typical pedestrian scenarios based on LTE specifications. Results are compared to those of other relay-assisted design approaches that have been recently proposed for the case of 1-antenna relay and the non-cooperative systems. For this comparison, we implement three propagation scenarios, with different SNR transmission conditions among links. Simulation results show that the availability of a bi-antenna array at the relay significantly improves the cooperative system performance, but improvements are highly dependent on the scenarios and relay protocol as well as of the cooperative second-hop quality.

Dependence of adaptive beamforming performance on directional channel modelled micro-cell scenarios

A beamforming algorithm, the conjugate gradient, is applied to a base station array, in the uplink, for UTRA-TDD. Street-type, micro-cell propagation scenarios are considered, using a wideband directional channel model. Groups of mobile users are placed at different distances from the base station, along the street axis. The average beamforming gain is analysed, varying the number of array elements, the distance of users from the base station, their number and grouping. The desired signal and interference-plus-noise powers evolve differently towards optimisation, due to the composition of the correlation matrix and the algorithms cost function. Allied to the high number of arriving correlated signals, this implies that a solution other than the MMSE one can lead to better interference suppression, for the tested scenarios. Among the tested scenarios, their influence highly depends on the number of users and their distances to the base station, the time-of-arrival spread overruling the importance of angle-of-arrival spread for larger numbers of users, closer to the base station, e.g., from 4 to 16 mobile users at 50 m or 1000 m from the base station, the beamforming gain can be degraded by nearly 4 dB or 2.3 dB, respectively, for 12 array elements.

Data-precoded algorithm for multiple-relay-assisted systems

A data-precoded relay-assisted (RA) scheme is proposed for a system cooperating with multiple relay nodes (RNs), each equipped with either a single-antenna or a two-antenna array. The classical RA systems using distributed space-time/frequency coding algorithms, because of the half-duplex constraint at the relays, require the use of a higher order constellation than in the case of a continuous link transmission from the base station to the user terminal. This implies a penalty in the power efficiency. The proposed precoding algorithm exploits the relation between QPSK and 4 L -QAM, by alternately transmitting through L relays, achieving full diversity, while significantly reducing power penalty. This algorithm explores the situations where a direct path (DP) is not available or has poor quality, and it is a promising solution to extend coverage or increase system capacity. We present the analytical derivation of the gain obtained with the data-precoded algorithm in comparison with distributed space-frequency block code (SFBC) ones. Furthermore, analysis of the pairwise error probability of the proposed algorithm is derived and confirmed with numerical results. We evaluate the performance of the proposed scheme and compare it relatively to the equivalent distributed SFBC scheme employing 16-QAM and non-cooperative schemes, for several link quality scenarios and scheme configurations, highlighting the advantages of the proposed scheme.

Novel precoded relay-assisted algorithm for cellular systems

Cooperative schemes are promising solutions for cellular wireless systems to improve system fairness, extend coverage and increase capacity. The use of relays is of significant interest to allow radio access in situations where a direct path is not available or has poor quality. A data precoded relay-assisted scheme is proposed for a system cooperating with 2 relays, each equipped with either a single antenna or 2-antenna array. However, because of the half-duplex constraint at the relays, relaying-assisted transmission would require the use of a higher order constellation than in the case when a continuous link is available from the BS to the UT. This would imply a penalty in the power efficiency. The simple precoding scheme proposed exploits the relation between QPSK and 16-QAM, by alternately transmitting through the 2 relays, achieving full diversity, while significantly reducing power penalty. Analysis of the pairwise error probability of the proposed algorithm with a single antenna in each relay is derived and confirmed with numerical results. We show the performance improvements of the precoded scheme, relatively to equivalent distributed SFBC scheme employing 16-QAM, for several channel quality scenarios.

Data precoded relay-assisted scheme for cellular systems

We propose a data precoded relay-assisted scheme, for a system cooperating with L relays, each one equipped with a single antenna. The use of relays is of significant interest to allow radio access in situations where either the direct path is not available or has poor quality. However because of the half duplex constraint at the relays, transmission of a data rate that would be possible using a modulation technique with m bits per symbol in the case a continuous link was available from the BS to the UT, requires the use of a constellation with 2m bits per symbol. This implies a penalty in the power efficiency. In this communication we propose a simple precoding scheme that exploits the relation between QPSK and 4L-QAM, by alternately transmitting through the L relays, keeping the diversity of order L. Numerical results are presented for the case where 2 relay nodes are used, for realistic scenarios with LTE parameters and using turbo coding, showing significant performance improvements relatively to the case of a relay based distributed Alamouti employing 16-QAM.

Using wideband directional channel models for spatial filtering in mobile communications

Separate but related studies have consistently shown how directional and wideband characteristics of the mobile propagation channel fundamentally establish physical constraints behind the spatial separation of signals. Their overall perspectives are hereby presented, extracting general guidelines to support the application of wideband directional propagation channel models. These aim at correctly and practically evaluating the performance of spatial filtering techniques. It is not the nature or the specificities of a model that are at stake, but how it should be applied. For the purpose of evaluating spatial processing techniques on a known channel model basis, it is argued that it should allow for statistical relevance, also involving sufficiently low implementation complexity; it should be characterised by physical quantitative measures of the channel richness; it should cover both macro-and microcell environments, for the same model nature, and possibly picocells; it ought to be applicable in the form of multiuser propagation scenarios, approximating a wide variety of practical interference situations; lastly, by being double-directional, its inherent nature should allow for its flexible and complete application to all types of single- or multiple-input, single- or multiple-output frameworks.